All RF transceiver ICs for mobile communication use an LC-VCO to generate the local oscillator (LO) signal, since this is the only topology that achieves the low phase noise required. Tuning the oscillation frequency is always done by changing the capacitance value, since it is almost impossible to change the inductance. The capacitance can be changed by using a varactor instead of a fixed capacitor, see FIG. 1.
Usually not a single large varactor is used, but instead a mixed discrete/continuous tuning scheme is used, see FIG. 2, in which                a smaller varactor is used to continuously change the VCO frequency as a function of the control voltage;        larger steps in frequency are realized by adding or removing digitally switched capacitors.There are two advantages to this band-switched approach:        the digitally switched capacitors often have a large Cmax/Cmin ratio, allowing a larger total VCO tuning range;        the VCO gain will be lower, allowing a better design of the PLL in which this VCO will be embedded.        
A VCO with a band-switched oscillator core is for example known from U.S. Pat. No. 6,836,193.
A problem that appears when very large tuning ranges are required, is that the operation of the oscillator core is not optimal for either the lower or the higher frequency bands.